From the customer's point of view, a flexible energy supply system only needs to function properly. However, this demand presupposes the perfect operation of all components, including the cables being used in this system. And this is exactly where problems came up in the early 1980s. Due to constantly – and frequently even tremendously – increasing loads resulting form the application of automation technology, guided cables often failed although the energy supply system itself was functioning perfectly. In extreme cases, failures caused by "corkscrews" and core ruptures brought the entire production process to a standstill and resulted in high costs. In order to find a solution to this unsatisfactory situation for its customers, igus® decided to take the initiative. As the first company worldwide, igus® began to develop complete energy chain systems. chainflex® cables and energy chains are now being offered as a delivery from a single source and with a system guarantee depending on the application in each case. Based on the increasing know-how gained since 1989 and on the very sophisticated series of tests that have been conducted since then, design principles were and are still being created that help prevent machine downtimes in factories throughout the world today.
Fleece
2Extruded, non-tension-proof center element
3Cores braided in layers
Single-wire bundles with short pitch lengths
2Center element for high tensile stresses
3Highly abrasionresistant, gussetfilled extruded jacket
Total shield with optimized braiding angle
2Gusset-filling extruded inner jacket
3Center element for high tensile stresses
Stranding in layers is significantly easier to produce and is therefore offered on the market in so-called "chain-suitable" cables at low cost. But what appears to be tempting at first glance can quickly turn into an expensive mistake when a "corkscrew" immobilizes the system being operated with these cables. How do these problems arise? A look at the cable structure can be quite helpful (see picture 1). In the case of stranding in layers, the cable cores are mostly stranded more or less firmly and relatively long in several layers around a center and are then provided with a jacket extruded to the form of a tube. In the case of shielded cables, the cores are wrapped up with fleece or foils. But what, for example, happens to a similarly structured 12-core cable during normal operation? The bending process compresses, in the movement of the core, the inner radius of the cable and stretches the core in the outer radius. Initially, this works quite well because the elasticity of the material is still sufficient. But very soon, material fatigue causes permanent deformations, and then, due to excursion from the specified paths, the cores make their "own compressing and stretching zones": The corkscrew is created, then followed rather quickly by core ruptures most of the time.
Stranding in bundles eliminates these problems by means of its very
sophisticated, multiply stranded internal structure. Here, the litz wires are
stranded with a special pitch length first and then the resulting cores are
stranded into single core bundles. For large cross sections, this is done
around a strain relief element. The next step is the renewed stranding of
this core bundle around a tension-proof center – a genuine center cord.
Due to this multiple stranding of the cores, all cores change the inner radius and the outer radius of the bent cable several times at identical spacing distances. Pulling and compressing forces balance one another around the high-tensile center cord that gives the stranded structure its necessary inner stability. Accordingly, the stranding remains stable even under maximum bending stress.
In principle, cable shields must fulfil two tasks:
Protecting the cables from external interferences
Shielding any interferences before transmitting them to the outside
Both tasks are equally important because faulty signals can cause considerable consequential damage in the system itself as well as in any external systems. Furthermore, this is an especially problematic point due to the fact that incorrect shielding usually cannot be detected from outside, and this is something that makes the trouble-shooting procedure extremely difficult. How can these kinds of problems arise in the first place?
Once again, the answer is to be found in the internal structure of the cable itself: Is the shielding designed for the movements of the cable? Although it may be very easy to shield a fixed cable, it is much more difficult to
guarantee the permanent shielding of a moving cable.
In the case of so-called "chain-suitable" cables, for example, the stranding bond of an intermediate layer is wrapped up with foils or fleeces. This stranding bond is supposed to guarantee the separation between
the cores and the shield braid. But something that functions quite well for the fixed installation of cables is often quite insufficient in the case of moving cables. This has to do with the fact that the foils and fleeces do
not create a bond between the stranding, shield and jacket and may fall apart under stress. Consequently, the metallic shield then rubs on the insulation of the cores – short circuits are then to be expected. But the production of the shield itself is very time-consuming and
cost-intensive and may have been the reason for the use of open braid shields or even simple wire wrappings. The disadvantages are quite obvious: Open shields only possess a limited shielding effect in their
moved state – motion and expansion reduce this effect even further. The type of shield is therefore an important point that is not even mentioned in some catalogues.
In its up to approx. 70% linearly and approx. 90% optically covered cables, igus® eliminates these weak points by means of an optimized internal
structure. In virtually all shielded chainflex® cables, a gusset-filled extruded inner jacket over the stranded structure is therefore used. This "second
jacket" fulfils two tasks:
It holds the stranded structure together and guides the individual cores as in a channel.
It serves as a firm, round base for a very tight-fitting shield.
The shield braid angle determined by long-term trials securely neutralizes the tensile strengths and is therefore ideally suited for e-chains®.
Due to the stable inner jacket, the shield cannot wander uncontrolled.
The shield itself exerts torsion protection on the braid.
Strain-relieving center
Stranding in bundles
Gusset-filled extruded inner jacket in shielded cables
Enclosed shield braid
Optimized shield braiding angle
Gusset-filled extruded jacket
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